Switching device using superlattice without any dielectric barriers

ABSTRACT

A switching device has an S (Superconductor)-N (Normal Metal)-S superlattice to control the stream of electrons without any dielectric materials. Each layer of said Superconductor has own terminal. The superlattice spacing is selected based on “Dimensional Crossover Effect”. This device can operate at a high frequency without such energy losses as devices breaking the superconducting state. The limit of the operation frequency in the case of the Nb/Cu superlattice is expected to be in the order of 10 18  Hz concerning plasmon loss energy of the normal metals (Cu; in the order of 10 3  eV).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a switching device using S-N-Ssuperlattice.

[0003] 2. Description of the Related Art

[0004] Since the discovery of the transistor effect by William BradfordShockley, John Bardeen and Walter. H. Brattain in 1948, thesemiconductor device for computing has generated a tremendous number ofconcepts related to the revolution of the world. The speed of processingdata is required to be highest to synchronize the stream ofconsciousness. In the field of electronic devices, in order to catch upwith such a stream, the circuit elements have been downsized to satisfythe “Scaling Rule” The rule has provided an appearance that theprocessing speed should be improved by downsizing the circuit assembly,including Field Effect Transistors (FET), to reduce the product ofresistivity and capacitance of integrated circuits. However, we arebecoming aware of difficulty of meeting contemporary demands of highspeed processing by the Scaling Rule. Since the switching operation ofthe FET is originated from motions of carriers neighboring the gatecontact by bias voltage, the switching frequency depends upon themobility of carriers. Recently Y. Nish predicted that the chip frequencywould be restricted approximately up to 1.1×10⁹ Hz until the year of2010 (Y. Nish, Proceedings of Internatinal Symposium on Control ofSingle Particles and its Application (1996)). According to theprediction, the performance of a Central Processing Unit (CPU) should bea little higher than the common units. That is to say that processinginflated amount of information by large-sized software should berestricted by the limit of the mobility of carriers insidesemiconductors.

[0005] On the other hand, a switching device using superconductingmaterial with a tunneling insulating barrier, predicted by Brian DavidJosephson, has been known to be a high frequency switching device, whichconsumes extremely low energy (B. D. Josephson, Phys. Rev. Lett.,1(7)(1962)251). However the Josephson junction device has never been putto practical use because the switching operation beyond a frequency of7×10⁸ Hz is suffered from chaotic noise. Besides, there are threeproperties such as attenuation of signals transmitted across thetunneling barrier, delay of signals by parasitic capacitance andmechanical fragility against thermal stress. The oxide superconductordiscovered by K. Alex Muller and J. Georg Bednorz (K. Alex Muller and J.Georg Bednorz, Zeitschrift fur Physik, B64(1986)189), which is able tooperate at higher temperature, has been introduced to the switchingdevice. In spite of a lot of trial, switching devices using an oxidesuperconductor have never been practically used because of their ownproperty. In this paper, we propose that we can solve all problems byusing a metal superconductor superlattice.

[0006] There are two reasons why the oxide superconductor has never beenapplied to practical Josephson devices. First, the phase change of thewave function should be fluctuated by the existence of incoherentinterfaces such as the grain boundaries. Second, it is difficult tointegrate the device because of the low transmittance rate of the wavefunction across such incoherent interfaces. Coherent length of the oxidesuperconductor is designed to be shortest (shorter than 0.1 nm) to raisethe superconducting critical temperature Tc (H. Hayakawa and Y. Takagi,Oyo Butsuri (in Japanese), 58(5)(1989)766). Such short coherent lengthis realized by inserting ionic layers with high electric polarization asthe partitions of the coherent region. Accordingly, the delay of wavefunction between conducting layers cannot be avoided (FIG. 1A). Besides,the delay should not be uniform in the region neighboring the incoherentinterface (FIG. 1B). By such rack of uniformity, thermal noises arepreference beyond the switching frequency of 10⁴ Hz (L. Hao, J. C.MacFarlane, C. M. Pegrum, Supercond. Sci. Technol., 9(1996)678) anddynamical impedance of the interface is increased (K. K. Likharev and VK. Semenov, JETP. Lett., 15(1972)3537), therefore, transmittance of thewave function is lowered in the high frequency region. This means thatthese problems cannot be essentially solved by partitioning the coherentregion by using dielectric material.

[0007] Superconductivity is decided by coherence of wave function of theCooper pair, and the coherence is realized by spin exchange correlation.This means we can solve the problem of partitioning the conductingregion by controlling spin exchange correlation. Then we are going totry to understand the mechanism of the Giant Magneto-Resistance (GMR) asan example of artificial control of spin exchange correlation. It isreported that GMR is realized in the metal system in a mesoscopic scaleand resistivity is lowered by 50% in the applied magnetic field (eg. M.N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P.Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett.,61(1988)2472). GMR is originated from the fact that transition ofitinerant electrons between the ferro-magnetic layers is restricted whenthe wave function considering spin direction is opposite to the nextlayer (Kondo effect (Jun Kondo, “An abstract of metal electron theory”(in Japanese), Shokabo Press (1983))). The experimental result on thespin ordering in the system revealing GMR has already been reported (N.Hosono, S. Araki, K. Mibu and T. Shinjo, J. Phys. Soc. Jpn.,59(6)(1990)1925). According to this report, the half-ordered reflection,which is the proof of spin ordering in the scale of the superlatticespacing, can be observed in the experiment of the neutron diffraction.

[0008] It should be emphasized that the well-controlled magnetic domaincan be realized in the mesoscopic system by control of the superlatticespacing. This suggests that transition of itinerant electrons can becontrolled by tuning the spacing of strongly correlated layers withoutinserting any dielectric insulator. According to this suggestion, we canalso design the superlattice using correlated materials such assuprconductor without any dielectric materials.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide superconductorwithout any dielectric materials.

[0010] Another object of the present invention is to provide a switchingdevice being operable at a high speed.

[0011] To achieve the objects, there is provided a superconductingswitching device having a Superconductor/Metal/Superconductorsuperlattice.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1A shows influence of electric polarization inside of theoxide superconductor upon the propagation of the wave function betweenthe conducting layers. In case of the Josephson device using the oxidesuperconductor, the thermal noise is preferential beyond the switchingfrequency of 10⁴ Hz. According to the report by K. K. Likharev, suchthermal noise is caused by the increase of dynamical impedance in thehigh frequency region. That is to say there exists a high amount ofelectric polarization inside the crystal of Perovskite structure.Furthermore the wave function should be delayed by electricpolarization.

[0013]FIG. 1B shows the schematic diagram showing the phase changeneighboring the incoherent interfaces. The ordering parameter shouldnaturally be lowered by the delay of the wave function and the couplingof the wave function should be weak.

[0014]FIG. 2 shows the layer-by-layer anti-ferromagnetic spinarrangement concluded by the experiment of the resistivity measurement.The electronic states in each layer were simulated by the ab-initioLinear Combination of Atomic Orbital calculation (Discrete VariationalXα approximation (H. Adachi, M. Tsukada and C. Satoko, J. Phys. Soc.Jpn., 45(1978)875)). Since spin fluctuation according to the mechanismof “thermal stabilization mechanism of the Cooper pair” 11 isindependent from the normal electronic state, we can treat themesoscopic spin arrangement schematically using the normal electronicstate. The spin direction in the next layer should be opposite in orderto lower the total energy described by the function of the exchangecorrelation parameter. If such anti-ferromagnetic arrangement isrealized in the system of the superconductor, the stream of electronsshould be divided without dielectric materials.

[0015]FIG. 3A shows the schematic diagram of unit cell of the 4-probeswitching device. The transition between the layers whose wave functionsconsidering spin directions are identical is preferential. As a resultof Kondo effect, the transition to the next layer is restricted.

[0016]FIG. 3B shows the schematic diagram of the 4-probe deviceconstructed by source layers, gate layers and drain layers. The switchshould be turned off, if Bloch resonance is modulated by the gateoperation or by single events from the external environments. Byconsidering the analogy between superconducting phenomena and GMR, thestream of electrons should be divided without any dielectric materials.This device could be made from another kind of spin-exchange-correlatedmaterials.

[0017]FIG. 4 shows the schematic diagram of the energy band inside ofthe unit cell of the proposed 4-probe device. As described previously,spin fluctuation according to the mechanism of “thermal stabilization ofthe Cooper pair” is independent from the normal electronic state,therefore, we can treat the mesoscopic spin arrangement schematicallyusing the normal state. The normal sate inside of the superconductinglayers are arranged anti-ferromagnetically in the mesoscopic scale andare separated by the electronic states of the normal metal layers. As aresult of Kondo effect and/or “Tunneling barrier effect (in the text)”,Bloch resonance is accomplished.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] In the system of Superconducting metal-Normalmetal-Superconducting metal (S-N-S) superlattice, the dimensionalcrossover effect has been reported. The group of Ivan K. Schuller et.al. has investigated the effect in the Nb/Cu system. According to theirreports (Cornel S. L. Chun, Guo-Guang Zheng, Jose L. Vincent, and IvanK. Schuller, Phys. Rev., B29(9)(1984)4915), the coherent length of theNb/Cu superlattice is anisotropic only under severe condition in themesoscopic system, and the correlation between superconducting layersshould be maxmal if the superlattice spacing is appropriated.

[0019] Recently we reported on the layer-by-layer anti-ferromagneticallyspin ordering in the Nb/Cu system on a mesoscopic scale (K. Tsukui, M.Yata, I. Ohdomari, T. Osaka, N. Yagi and H. Tsukui, Appl. Surface Sci.,162-163(2000)239-244). As the result of the resitivity measurements inthe system of the Nb/Cu superconducting superlattice whose spacing isfixed to be 16.8 nm and 14.7 nm, respectively, to realize the conditionthat the interlayer spin exchange correlation should be maxmal, ananomalous increase of the resistivity was observed with highreproducibility. Taking the fact into account that the anomaly wasobserved only in the case that the interlayer correlation was maxmal, weconcluded that the anti-ferromagnetic arrangement of the electron bandshould be realized in the mesoscopic scale and the anomaly should beascribed to the transition of the itinerant electrons described by Kondoeffect (FIG. 2).

[0020] If such anti-ferromagnetic ordering is also realized in asuperconductor system, the analogy of the superconducting phenomena andGMR should be concluded. As described above, the stream of electrons isdivided without a dielectric insulator in the system revealing GMR,therefore, the transition of Cooper pairs in the system of the S-N-Ssuperlattice should be controlled without any dielectric materials.

[0021]4-Probes Switching Device of the Preferred Embodiment

[0022]FIG. 3A shows the schematic diagram of a unit cell of a 4-probes(4-terminals) switching device. The transition of the electrons betweenthe correlated layers should be preferential and the transition to thenext layer should be restricted by the anti-ferromagnetic spinarrangement. Based on this philosophy, a 4-probe device with a source, agate and a drain has been proposed by the author (K. Tsukui). Theprinciple of switching operation is based on the modulation of the Blochresonance, (L. Esaki and R. Tsu, IBM research Note, RC-2418 (1969) L.Esaki and R. Tsu, IBM J. Res. Develop., 14(1970)61; L. Esaki, PhysicaScripta, T42(1992)103) and nonlinear transport due to Bloch resonance.As described in FIG. 4, the occupied electronic states with differentspin directions of superconducting layers are separated by normal metallayers in the same way as the semiconductor superlattice. Electronsinside of the superlattice are resonated and the resonance state couldbe modulated by the gate operation or single events introduced from theexternal environment. As a result of the modulation of Bloch resonance,the Source-Drain voltage has continuous values originated from thenonlinear transport property of the superlattice (K. F. Renk, E.Schomburg, A. A. Ignatov, J. Grenzer, S. Winnerl, K. Hofbeck, Physica,B244(1998)196), which is all the same as the case of the semiconductorsuperlattice except the treatment of tunneling barriers. In the case ofS-N-S structure, the itinerant electrons are transmitted through theparamagnetic metal layers. However the transition from the initialsuperconducting layer to the final superconducting layer is highlyrestricted by the spin direction at the lower temperature according toKondo effect. The resistivity of itinerant electrons (ρ↑) is decided bythe equation below.$\rho_{\uparrow} = {\rho_{0}{\pi^{2}\left( {N_{Para}\left( ɛ_{F} \right)} \right)}^{2}J^{2}{S\left( {S + 1} \right)}\left( {1 - {4{N_{Para}\left( ɛ_{F} \right)}J\quad \log \frac{k_{B}T}{D}}} \right)}$

[0023] The resistivity is a function of both temperature and electrondensity at the Fermi level of the inserted paramagnetic metal(N_(para)(ε_(F)). Transition probability of itinerant electron is verylow at low temperature below 100K, therefore, inserted paramagneticmetal plays the same role as insulator, which can be treated as the bandgap in the case of semiconductor superlattices. This “tunneling barriereffect” has been authorized to be adapted to the case of theparamagnetic layer with different angular momentum which is insertedbetween the spin-exchange-correlated layers (L. I. Schiff, “QuantumMechanics” (Second Edition), McGraw-Hill (1955)). Consequently thisdevice can be operated keeping the superconducting current by Blochresonance, therefore, the operation is independent of the frequencylimit decided by the gap energy in the case of the conventional RC-typeJosephson junction.

[0024] Furthermore the correlation between the source layers and thedrain layers can be summated to the unity because the basic units asmentioned before are stacked as the multilayer in the order ofSource(S)/Gate(G)/Drain(D)/G/S/G/D/G or SIG/DIS/GID as shown in FIG. 3B.For example, the sources are connected with each other, the gates areconnected with each other, the drains are connected with each other.Then the problem of the transmittance should be solved. Consequently,all problems toward the practical use of the superconductor device aresolved by using this device. It is possible to operate this device atextremely high frequency without such energy losses as the RC-typeswitching devices because superconducting state is not destroyed. Thelimit of the operation frequency in the case of the Nb/Cu superlatticeis expected to be in the order of 10¹⁸ Hz concerning plasmon loss energyof the normal metals (Cu; in the order of 10³ eV). If such highfrequency operation could be possible, the electrical pulse could betransformed to the photons and incident photons and high-energyparticles could be detected as electric pulse.

[0025] As stated above, we have discussed on the aspect that theoperation of the CPU at higher frequency by other ways than integratingthe circuits according to the Scaling Rule. In order to fabricateextremely high frequency switching devices using superconductors, weshould get rid of the parasitic capacitance (electric polarization)neighboring the junction. As an example of such a device, we proposed a4-probe device using the S-N-S superlattice to control the stream ofelectrons without any dielectric materials.

[0026] The contents of the cited references are incorporated herein byreference in their entirely.

[0027] This application is based on International Patent Application No.PCT/JP00/08143 filed on Nov. 17, 2000 and amendments under PatentCooperation Treaty articles 19 and 34 filed for the above-notedInternational Patent Application. The disclosure of the InternationalPatent Application and the amendments are incorporated herein byreference in their entirely.

What is claimed is:
 1. A superconducting switching device having aSuperconductor/Mtal/Superconductor superlattice.
 2. The superconductingswitching device according to claim 1, wherein each layer of saidSuperconductor has own terminal.
 3. The superconducting switching deviceaccording to claim 1, wherein the superlattice spacing is selected basedon “Dimensional Crossover Effect”.
 4. The superconducting switchingdevice according to claim 3, wherein said Superconductror comprises anNb layer, and said metal comprises a Cu layer, the thickness of the Nblayer and Cu layer are 16.8 nm and 14.7 nm, respectively.
 5. Thesuperconducting switching device according to claim 4, wherein thesuperlattice comprises a superconductiong layer with a source terminalserving as a source layer, a superconducting layer with a gate terminalserving as a gate layer and a superconducting layer with a drainterminal serving as a drain layer.
 6. The superconducting switchingdevice according to claim 5, wherein the superconducting layers arestacked by inserting metal layers.
 7. The superconducting switchingdevice according to claim 5, wherein this switching device operates bymodulating the Bloch resonance inside of the superlattice by theelectronic state of the gate layer and/or single events introduced fromthe external environment.
 8. The superconducting switching deviceaccording to claim 1, wherein this switching device operates bymodulating the Bloch resonance inside of the superlattice by theelectronic state of the gate layer and/or single events introduced fromexternal environment.
 9. The superconducting switching device accordingto claim 4, wherein a Source-Drain voltage reveals one of multi-value bythe non-linear electron transportation in case of the Bloch resonance.10. The superconducting switching device according to claim 1, whereinsaid superlattice includes a plurality of unit cells each having thesuperconductiong layer serving as a source layer, a superconductinglayer serving as a gate layer and a superconducting layer serving as adrain layer.
 11. The superconducting switching device according to claim10, wherein said superlattice has a structure in which the source layer,the gate layer, the drain layer, the gate layer, the source layer, thegate layer, the drain layer, and the gate layer are stacked in thisorder, or a structure in which the source layer, the gate layer, thedrain layer, the gate layer, the drain layer, the gate layer, and thesource layer stacked in this order, and the source layers are connectedwith each other, the drain layers are connected with each other, andgate layers are connected with each other.
 12. A switching devicecharacterized by having a Superconductor/Metal/Superconductorsuperlattice structure and terminals attached to the superconductors,and operating based on principle of the modulation of the Blochresonance.
 13. A superconducting switching device having a latticehaving stacked first Superconductor layer, a first metal layer, a secondSuperconductor layers, a second metal layer, and a third Superconductorolayer, each of said first to third Superconductor layer having ownterminal, thickness of the first to third Superconductor layers andfirst and second metal layers are selected based on “DimensionalCrossover Effect”.
 14. The superconducting switching device according toclaim 13, wherein the first superconductiong layer serves as a sourcelayer, the seocnod superconducting layer serves as a gate layer and thethird superconducting layer serves as a drain layer.
 15. Thesuperconducting switching device according to claim 13, wherein thisswitching device operates by modulating the Bloch resonance inside ofthe superlattice by the electronic state of the gate layer and/or singleevents introduced from the external environment.
 16. The superconductingswitching device according to claim 15, wherein a Source-Drain voltagereveals one of multi-value by the non-linear electron transportation incase of the Bloch resonance.
 17. The superconducting switching deviceaccording to claim 13, wherein the source layer, the gate layer, thedrain layer, the gate layer, the source layer, the gate layer, the drainlayer, and the gate layer are stacked in this order, or a structure inwhich the source layer, the gate layer, the drain layer, the gate layer,the drain layer, the gate layer, and the source layer are stacked inthis order.